Allow adding th title attribute

It would be useful to have a `title` attribute added to `th` elements. This attribute is used by the browser to set a tooltip on hover.

Although it's possible to add a custom `thead`, it seems to me that this breaks the automatic generation of sorting and filtering logic.

I followed the idea behind `data-header-class` attribute and created a `data-header-title` attribute. In addition, if no `data-header-title` is provided, the `data-title` attribute value is used by default.